A tube isothermal reactor essentially comprises an outer shell, a suitable drum containing the catalytic bed, a tube bundle, one or two tube plates for U-shaped or straight tubes, respectively. In radial or axial-radial flow embodiments, the catalytic bed has an annular structure essentially defined by two coaxial cylindrical walls, termed inner collector and outer collector; as a consequence, the tube bundle also has an annular structure.
The tubes are fed with a heat exchange fluid, for example water or steam, which releases or removes heat from the bed, allowing the temperature of the bed itself to be controlled. For this reason the reactor is also defined isothermal. For example, in reactors for the methanol synthesis, the chemical reaction is exothermal and the tube bundle is typically passed through by boiling water.
Basically, the reactor has a shell side which contains the catalyst and is passed through by reagents and reaction products, usually gaseous, and a tube side (inside of the tubes) passed through by said heat exchange fluid. It should be noted that the catalyst becomes exhausted over the time and must be periodically unloaded to be regenerated or replaced with fresh catalyst, which requires access to the shell side of the reactor.
It is known that the tube plates are a costly item in reactors of this type. Said plates are in the form of a disk or ring with an outer diameter substantially equal to the diameter of the reactor; they have a considerable thickness in order to withstand pressure and, consequently, require a large quantity of high-quality material (alloyed steel). Moreover conventional tube plates make the access to the shell side of the reactor difficult for loading and unloading the catalyst, which results in these operations being time-consuming and expensive.
In order to eliminate the above costly item, tube isothermal reactors have been proposed where the tube plates are replaced by a first body acting as a fluid distributor and by a second body acting as a collector, the ends of the tubes being welded to said first and second body. Said bodies have for example a toroidal, spherical, cylindrical or ellipsoidal shape and are smaller than the cross-section of the apparatus so as to allow the loading and unloading of the catalyst.
The distributor and the collector so realized are lighter and less costly than a tube plate, however they give rise to a series of drawbacks.
A first drawback is that the tubes, which are regularly and uniformly distanced (e.g. square or triangular configuration) in the central part of the bundle, must be arranged close to each other in proximity of the distributor body and the collector body, resulting in each single tube having one or more bends at its ends; these bends are all different from each other, depending on the radial position of the tube inside the reactor. This feature poses a number of problems from the point of view of the constructional design and requires special bending machines to obtain precise and repeatable geometrical forms.
Another problem is that the engagement of the tubes on the aforementioned bodies occurs in directions perpendicular to the surfaces of said bodies, which generally do not coincide with the axis of the tubes. In order to mount the tubes within the limited space available during the assembly of the apparatus, it is therefore necessary to provide, at least at one end of each tube, an intermediate joint between the shaped end and the straight part. This increases the costs and gives rise to possible leakage points.
A leakage due to a welding defect of one of these intermediate joints would be very difficult to detect, and cannot be repaired owing to the large number of tubes close to each other. Basically the only solution is to seal the damaged tube.
As mentioned above, the distributor body and the collector body are smaller than the cross-section of the apparatus so as to allow the inflow/outflow of catalyst during loading/unloading thereof. However, the closer arrangement of the tubes around said bodies constitutes an obstacle for the outflow of the catalyst and partly cancels the advantage obtained. Moreover the ends of the various tubes, which are all different in terms of path, height and length, hinder the correct distribution of the heat exchange fluid on the tube side, introducing non-uniform head losses; consequently the flow inside the tubes may not be uniform and some tubes in the bundle may have a reduced cooling or heating capacity. All this constitutes a drawback for the process.
Where the tubes deviate from the straight and parallel configuration, owing to the curved ends, zones are inevitably created where the tubes are arranged closer to each other or are spaced apart with respect to the design layout; these zones are undesired non-standard features giving poor contribution to heat exchange. For this reason, said tube ends, although extending lengthwise inside the apparatus, are not taken into account when determining the heat exchange surface and therefore adversely affect the filling coefficient of the apparatus, i.e. the useful exploitation of its internal volume.
These drawbacks may significantly reduce the cost savings achieved by using the aforementioned collectors and distributors instead of the tube plates.
In order to solve the problem of accessibility to the shell side (hindered by the tube plates) it has also been proposed reducing the dimensions of said plates, i.e. for example using plates having a cross-section which is substantially smaller than the bed. This solution, however, also requires bending of the tubes so as to cause them to converge onto the plate section, with the added drawback that, compared to the situation described above, they require even a double bend for each end. In addition to the cost aspects already mentioned above, it should be noted that the minimum distance between the tubes is present in the region of the tube plate, which means that it is necessary to use a bundle of tubes which are relatively spaced from each other along the reactor, and therefore apparatuses with very densely occupied exchange surfaces cannot be obtained.
Basically, the prior art does not offer yet a convenient solution for providing a low-cost tube isothermal reactor.
EP 2 246 109 discloses a tube isothermal chemical reactor with concentric ranks of tube packs.